It is well known to DSL engineers that crosstalk interactions among the twisted copper pairs in a binder group present one of the biggest performance bottlenecks in modern DSL systems. Crosstalk is generated because of electromagnetic coupling among the pairs and can be due to near-end (NEXT) or far-end (FEXT) transmissions as shown in FIG. 1.
Referring in particular to FIG. 1, lines 120, 130, 140 and 150 are carried within a binder 110. Line 1 (120) and line 2 (130) experience near-end crosstalk 160. Line 2 (130) and line 3 (140) experience far-end crosstalk 170. In each situation, the signal-to-noise ratio decreases (more noise) and transmission quality degrades.
Several methods have been devised to control the detrimental effects of crosstalk and allow multiple services to co-exist in a binder group. They are based on limiting either the power or the bandwidth that the transceiver uses, so that its harmful effects on other services are within acceptable limits. In North America this set of guidelines has been compiled into an ANSI-T1 spectrum management standard.
Spectral management guidelines are useful in policing harmful emissions in the network but they do not actively control or compensate for the effects of crosstalk. Even with those guidelines, crosstalk still is the number one performance impairment for all services in the binder. Novel technologies that hold significant promise in combating crosstalk impairments irrespective of the spectral masks used, are based on joint processing of multiple signals from many pairs and come under the name Multi-Input-Multi-Output (MIMO) processing or vectored transmission.
MIMO techniques have been proposed for many diverse applications where information is transmitted through multiple channels which interact with each other. Examples include wireless LAN (IEEE 802.11) and wireless access (IEEE 802.16) systems as well as 10G Ethernet transceivers. Their application to the local loop network holds the promise of mitigating the crosstalk bottleneck and offering a new level of performance for copper based services.
The local loop however, offers a number of challenges to the introduction of these technologies. Contrary to other areas where MIMO has been successfully applied (e.g., wireless), the copper network always has to contend with legacy equipment and its effect on the new technologies. Only in very few cases (like in VDSL deployments) can one reasonably expect a “greenfield” application.
In most other cases, “forklift upgrades” are prohibitively expensive and new technologies have to be introduced in an evolutionary fashion. Furthermore, the structure of the local loop network itself and the deployment practices of the carriers have to be well understood in order to incorporate the technology in appropriate equipment, services and deployments that will make its introduction a success.
Thus, it may be useful to examine MIMO technologies from an outside plant point of view. The signal processing details of the technology need not be fully described to understand the challenges inherent in the local loop portion of networks. The effects on network planning, provisioning and deployment practices of MIMO deployment may produce a barrier to entry for this technology, and it may be useful to minimize these effects.